DESCRIPTION: The long-term goal of this project is to understand the auditory processes that determine performance under conditions of stimulus uncertainty, that is, when one or more of the physical parameters of the sounds cannot be predicted. Stimulus uncertainty nearly always degrades performance, sometimes by several orders of magnitude. Previous work by this laboratory on effects of masker uncertainty, signal uncertainty, and combinations of signal and masker (context) uncertainty supports the hypothesis that relative stimulus variability is a major determinant of performance with uncertain stimuli. In the proposed studies, the first specific aim is to continue tests of whether stimuli with greater variability dominate performance, and to provide direct tests of the variability-based CoRe model of uncertainty effects. The four remaining specific aims examine higher-order, listener-based processes known to influence performance with complex stimuli. These aims will test: 1) whether manipulations that promote sound segregation reduce effects of frequency uncertainty; 2) whether general listening strategies differ across individuals and tasks; 3) whether listeners "weight" stimulus components of equal variability unequally, and whether non-optimal weights degrade performance; and 4) whether manipulations of listener expectations by instructions and payoff matrices can change performance. Experiments are grouped in four categories based on type of uncertainty and common measurement procedures: 1) studies of masker uncertainty using detections tasks; 2) studies of signal uncertainty using probe-signal tasks commonly used to assess higher-order processes; 3) studies of signal uncertainty using sample-discrimination tasks, in which relative stimulus variation can be easily quantified; and 4) studies of combined signal and context (masker) uncertainty using sample-discrimination tasks. Because stimulus uncertainty is inherent in everyday listening situations, these experiments will contribute to more realistic models of auditory processing that include important central processes, such as listener strategies and decision rules, as well as peripheral, stimulus-based processes.